Also, because French's indefinite articles for uncountables or undetermined plurals are built on a genitive / ablative, you will find:

As-tu du pain ? Oui, j'en ai.
As-tu des pommes? Oui, j'en ai.

What is more, it can be used with the undetermined article un or rather with cardinal determiners like un, deux, trois, etc. In which case you have to understand the implicit genitive il a pris une pomme (indéterminée parmis les pommes qui existent).